1. Field of the Invention
The present invention relates to a discharge tube, or more in particular, to a discharge tube comprising at least a pair of electrodes with the discharge surfaces thereof arranged in opposition to each other in a space filled with a sealing gas.
2. Description of the Related Art
The lighting circuit of a gas discharge lamp such as a metal halide lamp or a xenon lamp used as a high-voltage lamp includes a discharge tube as shown in FIG. 5 or 6 (a switching spark gap, hereinafter sometimes referred to as "SSG") for supplying the operating voltage to the lamp to be turned on.
The SSG 10 shown in FIG. 5 includes a cylindrical member 12 made of an insulating material such as a ceramic, and a pair of electrodes 14a, 14b arranged with discharge surfaces 16, 16 thereof inserted in a space portion 22 by way of the openings at the two ends of the cylindrical member 12. The discharge surfaces 16, 16 are arranged in opposition relation to each other through a sealing gas filled in the space portion 22. The flat portions of the discharge surfaces 16, 16 are coated with insulating layers 18, 18, respectively, of an insulating material.
The SSG 10 shown in FIG. 6 has a substantially similar structure to the SSG shown in FIG. 5, except that recesses 20, 20 are formed in the discharge surfaces of a pair of the electrodes 14a, 14b. The surfaces of the recesses 20, 20 are coated with an insulating material thereby to form insulating layers 18, 18, respectively. The area of the discharge surface can be enlarged and the service life of the discharge tube can be lengthened by forming the recesses 20, 20 in the discharge surfaces in this way.
In the case where the SSG 10 shown in FIGS. 5 and 6 is to be discharged continually, it is necessary to supply a specific operating voltage at a frequency of several ms to several tens of ms in stable fashion to the lamp, etc. For this purpose, JP-A-9-22769 proposes a SSG in which a pair of the insulating layers 18, 18 formed on the discharge surfaces 16, 16 of a pair of the electrodes 14a, 14b are formed of an insulating material mixed with at least an alkali metal salt selected from potassium bromide, potassium fluoride and sodium fluoride.
When the SSG proposed in the aforementioned patent publication is discharged continually at intervals of 200 Hz (5.0 ms) in frequency, as shown in FIG. 7, a predetermined operating voltage can be supplied in a stable fashion. The SSG exhibiting the discharge characteristic of FIG. 7 is the SSG 10 shown in FIG. 5 having the insulating layers 18 mixed with potassium bromide. The potassium bromide thus added represents 15% by weight of the water glass solution forming the insulating layers 18. This potassium bromide is dissolved in the water glass contained in the insulating material and coated on the discharge surfaces 16, 16 of the electrode pair 14a, 14b.
In FIG. 7, point A represents a discharge start voltage, which is set to 1000 V in this case.
In recent years, gas discharge lamps such as metal halide lamps or xenon lamps have been employed for home-use projectors or TVs or for headlights of automobiles. The SSG is used for the lighting circuit of such lamps. For automotive applications, the SSG is often installed in the engine compartment.
For this reason, demand is high for a SSG which can discharge in stable fashion even when discharged continually at intervals of 400 Hz (2.5 ms) in a high-temperature (150.degree. C.) environment.
In the case where the SSG 10 formed with an insulating layer 18 made of potassium bromide, representing 15% by weight of a water glass solution, is discharged continually at intervals of 2.5 ms at room temperature, however, the discharge is often suspended midway as shown in FIG. 8.
Also in the case where the SSG 10 is continually discharged at intervals of 200 Hz (5.0 ms), it has been found that when the ambient temperature of the SSG 10 is increased to 150.degree. C., the discharge start voltage is liable to become unstable as shown in FIG. 9.